Predicting the Introduction and Transmission of Rift Valley
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PREDICTING THE INTRODUCTION AND TRANSMISSION OF RIFT VALLEY FEVER VIRUS IN THE UNITED STATES A Thesis by ANDREW JOHN GOLNAR Submitted to the Office of Graduate and Professional Studies of Texas A&M University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Chair of Committee, Gabriel Hamer Committee Members, Robert Coulson Bret Collier Head of Department, David Ragsdale December 2014 Major Subject: Entomology Copyright 2014 Andrew John Golnar ABSTRACT Rift Valley fever virus (RVFV) is a mosquito-borne virus in the family Bunyaviridae that has spread throughout continental Africa to Madagascar and the Arabian Peninsula. The establishment of RVFV in North America would have serious consequences for human and animal health in addition to a significant economic impact on the livestock industry. Specific objectives of this thesis are to identify high-risk regions involved in RVFV importation to the U.S., evaluate pathways of introduction, and theoretically quantify the relative importance of local vectors and vertebrate hosts to RVFV transmission should the virus reach the U.S. To estimate the relative risk of RVFV introduction to the U.S., the number of infectious mosquitoes arriving in the U.S. was quantified for five pathways: infected mosquitoes arriving by airplane, infected mosquitoes arriving by boat, infected mosquitoes arriving through tire trade, infected humans arriving by flight, and the trade of infected mammals. Results suggest that mosquito transport by airplane, mosquito transport by ship, and human travel are important pathways for RVFV introduction to the U.S. New York, Houston, Washington D.C., and Atlanta are high-risk regions for RVFV introduction in the U.S. Further, Saudi Arabia, South Africa, Nigeria, Egypt, Senegal, Ethiopia, Yemen and Angola are identified as regions at-risk for importing RVFV to the U.S. Published and unpublished data on RVFV vector competence, vertebrate host competence, and mosquito feeding patterns from the United States were combined to ii quantitatively implicate mosquito vectors and vertebrate hosts that may be important to RVFV transmission in the United States. A viremia-vector competence relationship based on published mosquito transmission studies was used to calculate a vertebrate host competence index which was then combined with mosquito blood feeding patterns to approximate the relative contribution of a mosquito or vertebrate host to RVFV transmission. Results implicate several Aedes spp. mosquitoes and vertebrates in the order Artiodactyla as important hosts for RVFV transmission in the U.S. Moreover, this study identifies critical gaps in knowledge necessary to comprehensively evaluate the different contributions of mosquitoes and vertebrates to potential RVFV transmission in the U.S. iii DEDICATION I dedicate this work to my family and friends who are tremendously supportive and patient while I continue to piece together my understanding of life, death and everything in-between. iv ACKNOWLEDGEMENTS First and foremost, I want to thank my committee for their guidance and support. I want to thank my committee chair, Dr. Gabe Hamer, for his perpetual enthusiasm and expertise in the field of disease ecology. I want to thank all the members of the Texas A&M Hamer lab for their availability, constructive feedback, and enthusiasm for disease ecology. I thank Dr. Gabe Hamer, Dr. Michael Turell, Dr. Desiree LaBeaud, and Dr. Rebekah Kading for their time and expertise to develop and publish Chapter III. I want to thank Samantha Casas for consolidating data from published literature and Dr. John Morrill for providing unpublished data. I want to thank the National Science Foundation Graduate Research Fellowship program #1252521 and the National Science Foundation/National Institute of Health Ecology of Infectious Disease program #EF- 0840403 for funding. Last of all, I want to thank my family and friends who help keep what matters most in perspective. v TABLE OF CONTENTS Page ABSTRACT .......................................................................................................................ii DEDICATION .................................................................................................................. iv ACKNOWLEDGEMENTS ............................................................................................... v TABLE OF CONTENTS .................................................................................................. vi LIST OF FIGURES ........................................................................................................ viii LIST OF TABLES ............................................................................................................ ix CHAPTER I INTRODUCTION AND LITERATURE REVIEW ................................... 1 CHAPTER II QUANTIFYING PATHWAYS OF RIFT VALLEY FEVER VIRUS INTRODUCTION TO THE UNITED STATES ............................................................... 4 Introduction ......................................................................................................... 4 Methods .............................................................................................................. 9 Model Assumptions .......................................................................................... 10 Host Movement Data ........................................................................................ 11 Mosquito Importation ....................................................................................... 14 Infected Vertebrates .......................................................................................... 17 Infected Humans ............................................................................................... 19 Frequency of Invasion in U.S. Cities ................................................................ 20 Results............................................................................................................... 21 Discussion ......................................................................................................... 25 Conclusion ........................................................................................................ 28 CHAPTER III PREDICTING THE MOSQUITO SPECIES AND VERTEBRATE SPECIES INVOLVED IN THE THEORETICAL TRANSMISSION OF RIFT VALLEY FEVER VIRUS IN THE UNITED STATES .................................................. 30 Introduction ....................................................................................................... 30 Methods ............................................................................................................ 33 Results............................................................................................................... 42 Discussion ......................................................................................................... 50 CHAPTER IV CONCLUSION ........................................................................................ 59 REFERENCES ................................................................................................................. 61 vi Page APPENDIX A: SUPPLEMENTAL MATERIAL ........................................................... 73 vii LIST OF FIGURES Page Figure 2.1. Countries in Africa and the Arabian Peninsula implicated to have high risk connectivity with the U.S. in the context of RVFV importation ............. 23 Figure 2.2. The number of RVFV infectious mosquitoes days in the U.S. per year ........ 25 Figure 3.1. Dose-dependent relationship between exposure viremia and vector competence ..................................................................................................... 36 Figure 3.2. A graphical representation of the mean viremia profiles demonstrated by 20 different vertebrates after exposure to virulent strains of Rift Valley fever virus ....................................................................................................... 37 Figure 3.3. Rift Valley fever virus host competence index values for 20 vertebrate hosts based on experimental infection studies characterizing viremia profiles in PFU/ml or TCID50 ......................................................................... 45 viii LIST OF TABLES Page Table 2.1. Range of parameter estimates based on published literature ......................... 22 Table 2.2. Estimated risk of Rift Valley fever virus introduction to the United States .............................................................................................................. 22 Table 3.1. Estimated dissemination rate, transmission rate, and vector competence for mosquitoes exposed to 7.5 log PFU/ml of Rift Valley fever virus ........... 43 Table 3.2. Relative risk of mosquitoes contributing to Rift Valley fever enzootic transmission in the U.S. .................................................................................. 47 ix CHAPTER I INTRODUCTION AND LITERATURE REVIEW Globalization and the movement of people and goods worldwide is reshaping global ecosystems and facilitating the spread of pathogens (Tatem and Tatem 2006, Hatcher et al. 2012). With the discovery of 100 new viral diseases in the past 30 years, pathogen dispersal is moving to the forefront of the public health arena (Daszak et al. 2000, Gubler 2002, Apperson et al. 2004). Biological invasions are associated with a variety of adverse affects and are often irreversible once established (Simberloff 2005). For this reason, it is important